Examples of indexable drills used for forming holes in metal include drills disclosed in Patent Documents 1 and 2 given below and drills shown in FIGS. 16 and 17.
In these drills, a pair of plate-shaped indexable inserts 5 (see FIGS. 16 and 17 for the numeral) having a clamp hole at the center are used in combination with each other. The indexable inserts 5 are arranged at an end of a drill main body 1 at positions where distances from the drill center differ from each other and at phases different from each other by 180° in the circumferential direction of the drill. The indexable insert at the inner side of the drill forms a center cutting edge 8 and the indexable insert at the outer side of the drill forms a peripheral cutting edge 9.
In this type of drill, generally, the drill diameter is changed (drills having different outer diameters are formed) using indexable inserts with the same specification as follows. That is, as shown in FIG. 18, the drill diameter (½D in the figure shows the radius of the drill) is changed by moving the peripheral cutting edge 9 in the radial direction while the center cutting edge 8 is fixed.
In addition, when the edges engage a workpiece, a central section of the center cutting edge 8 or a central section of the peripheral cutting edge 9 engages the workpiece first.
Patent Document 1: Japanese Patent No. 2704917
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-235606